the air intake breathes out of the passenger side fender so it has a vent to allow the air to flow better. the drivers side doesnt have it because it doesnt need it.

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To add to this, as the truck is driveing down the road, air from the spinning tire helps to "push" air into that opening, creating a higher pressure area in that fender to assist in feeding air into the air box. This has been done on many vehicles in the past in different wheel well openings, or in front of them, not only to aid air box efficiency, but for other things like brake cooling (Porsche, Mercedes, etc.) aerodynamics, etc.